Breast cancer and other breast lesions continue to be a significant threat to women's health. X-ray mammography currently is the most widely used tool for early detection and diagnosis, and is the modality approved by the U.S. Food and Drug Administration to screen for breast cancer in women who do not show symptoms of breast disease. A typical x-ray mammography system compresses and immobilizes a patient's breast on a breast platform positioned between an x-ray source and an x-ray imager, and takes a projection x-ray image (called here a conventional mammogram or simply mammogram) using a collimated cone or pyramid beam of x-rays at appropriate factors such as mA (current), kVp (voltage) or keV (energy), and msec (exposure time). In the United States, typically two views are taken of each breast, one from above (cranial-caudal, or CC, with the image plane generally at a 0° angle to the horizontal and one from the side (mediolateral-oblique, or MLO, with the image plane at an angle of typically around 45° to the horizontal). Different typical views may be taken in other countries. The x-ray source typically is an x-ray tube operating at or in the neighborhood of 25-30 kVp, using a molybdenum, rhodium, or tungsten rotating anode with a focal spot of about 0.3 to 0.4 mm and, in some cases, 0.1 mm or less. An anti-scatter grid between the breast and the imager can be used to reduce the effects of x-ray scatter. The breast is compressed to reduce patient motion and also for reasons such as reducing scatter, separating overlapping structures in the breast, reducing the x-ray thickness of the imaged breast and making it more uniform, and providing more uniform x-ray exposure. Traditionally, the imager has been a film/screen unit in which the x-rays impinging on the screen generate light that exposes the film. In the last several years, mammography systems using electronic digital flat panel array receptors have made significant inroads. A Selenia™ digital mammography system with such a digital flat panel x-ray receptor or imager is offered by Lorad, a division of the assignee hereof, Hologic, Inc. of Bedford, Mass. See brochure “Lorad Selenia™” Document B-BI-SEO US/Intl (5/06) copyright Hologic 2006. Digital mammography has significant advantages and in time may fully supplant film/screen systems. Additional information regarding digital mammography systems and processes offered by the common assignee can be found at <www.hologic.com>. Digital tomosynthesis also has made advances and the assignee hereof has exhibited breast tomosynthesis systems at trade shows and has carried out clinical testing. It is a three-dimensional process in which several two-dimensional projection views are acquired at respective different angles but lower x-ray dose than conventional mammograms, and are reconstructed into tomosynthesis slice views that can be along any desired plane in the breast. For tomosynthesis, the breast is still immobilized but may be compressed to the same or lesser extent than in conventional mammography. See, e.g., International Application WO 2006/058160 A2 published under the Patent Cooperation Treaty on Jun. 1, 2006 and Patent Application Publication No. 2001/0038681 A1, PCT application International Publication No. WO 03/020114 A2 published Mar. 13, 2003, U.S. Pat. Nos. 7,142,633, 6,885,724, 6,647,092, 6,289,235, 5,051,904, 5,359,637, and 4,496,557, and published patent applications US 2004/0109529 A1, US 2004/0066884 A1, US 2005/0105679 A1, US 20050129172A1, and Digital Clinical Reports, Tomosynthesis, GE Brochure 98-5493, 11/98. Reference markers can be used in x-ray imaging for purposes such as checking the rotation angle and unwanted shift of center of rotation of an x-ray source and receptor (imager), and fiducial phantoms can be used in 3D angiography to calibrate for irregular scan geometries. See, e.g., U.S. Pat. Nos. 5,051,904, 5,359,637, and 6,289,235, N. Navab, et al., Dynamic geometrical calibration for 3D cerebral angiography, SPIE Vol. 2708, pp. 361 370, and said PCT published application WO 03/020114 A2. A tomosynthesis system specifically for imaging patients' breast is disclosed in commonly owned U.S. Pat. Nos. 7,123,684 and 7,245,694. The same system can be selectively used for mammography and tomography, in the same or different compressions of the patient's breast.
It is desirable to know the geometric thickness of the immobilized breast in both film/screen and digital flat panel x-ray mammography as well as in tomosynthesis in order to make appropriate setting for the imaging procedure, such as settings for the x-ray tube that control the x-ray beam. Knowing the breast thickness can also help in quantitative assessments regarding x-ray images of the breast, such as in assessing the nature and clinical significance of x-ray attenuation properties of the breast. It can also be used in tomosynthesis reconstructions such as to determine the required reconstructed field of view or desired display field of view. It has been proposed to use encoders to measure the geometric height of a breast compression paddle and use the result to estimate the geometric thickness of the breast but this process typically has a relatively high error because of factors such as tilting and geometric distortions of the paddle as it compresses the breast, and because the encoders must be calibrated. The information on the paddle height is oftentimes stored in the DICOM header associated with the medical image, and this information is used for several purposes, including the calculation of body part radiation exposure. It also has been proposed to derive breast thickness information from measuring the paddle compression force versus time and to use the results to guide control factors such as kV, mAs, and filter selection, as in commonly assigned U.S. Pat. No. 7,123,684. The geometric thickness of any other body part being x-rayed also can be of interest, and one process for basing an estimate on non-contact ranging using ultrasound is discussed in U.S. Pat. No. 4,597,094. It has also been proposed to use a calibration phantom with embedded pellets of high-density material serving as markers and to use the imaged markers in calibrating the system, such as once a week. See U.S. Pat. No. 7,142,633 cited above.
Proposals have been made for automated methods of estimating breast density from mammograms, including volumetric and areal estimates. One example, using digitized film mammograms, is given in Br J Radiol. 2006 May; 79 (941):378-82 16632617. These methods are understood to derive the volume of fibroglandular tissue in the breast tissue for a cross-sectional area in a mammogram. The accuracy of these methods is believed to be related to the accuracy of measurement of the actual compressed breast. In addition, information has been published proposing that breast density is related to the breast cancer risk, See Cancer Epidemiology Biomarkers & Prevention, Vol 13, 715-722, May 2004, American Association for Cancer Research.
The patents and other publications identified above, including the brochure “Lorad Selenia™” and said published application WO 2006/058160 (corresponding U.S. patent application Ser. No. 11/791,601), are hereby incorporated by reference in this patent specification.